A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions; and a pair of electrodes arranged on both surfaces of the polymer electrolyte membrane. The electrode is usually composed of a catalyst layer comprising a carbon powder carrying a platinum group metal catalyst, and a hydrogen ion conductive polymer electrolyte; and a gas diffusion layer having both gas permeability and electronic conductivity, which is formed on the outer surface of the catalyst layer and made of, for example, carbon paper subjected to water repellent treatment.
In order to prevent the supplied gas from leaking out and prevent the fuel gas and oxidant gas from mixing together, a gas sealing material or gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane therebetween. The sealing material or gaskets are combined integrally with the electrodes and the polymer electrolyte membrane in advance. This is called “MEA” (electrolyte membrane and electrode assembly). Disposed outside the MEA are conductive separator plates for mechanically securing the MEA and for electrically connecting adjacent MEAs in series. The separator plates have, in the portions in contact with the MEA, a gas flow channel for supplying a reaction gas to the electrode surface and for removing a generated gas and an excess gas. Although the gas flow channels may be provided separately from the separator plates, grooves are generally formed on the surfaces of the separator plates to serve as the gas flow channels.
As a catalyst layer of a polymer electrolyte fuel cell, in general, a thin sheet formed from a mixture of a hydrogen ion conductive polymer electrolyte and a carbon fine powder carrying a platinum group metal catalyst is used. At present, as the hydrogen ion conductive polymer electrolyte, a perfluorocarbon sulfonic acid is generally used. The catalyst layer is formed by mixing a carbon fine powder carrying a catalyst such as platinum with a dispersion of a polymer electrolyte, which was prepared by dispersing a polymer electrolyte in an alcohol based solvent such as ethanol, and adding an organic solvent having a relatively high boiling point such as isopropyl alcohol and butyl alcohol to the mixture so as to form an ink; and applying the ink using a screen printing method, a spray coating method, a doctor blade method, or a roll coater method.
In the catalyst layer of the polymer electrolyte fuel cell, the size of the reaction area of a three-phase interface consisting of pores serving as reaction gas supply channels, a polymer electrolyte having hydrogen ion conductivity and an electrode material of an electron conductor is one of the most important factors that affect the discharge performance of the cell.
In order to increase the three-phase interface, conventionally, attempts were made to provide the interface between a polymer electrolyte membrane and a porous electrode with a layer in which the electrode material and the polymer electrolyte are mixed and dispersed. For instance, Japanese Examined Patent Publication Nos. Sho 62-61118 and Sho 62-61119 propose a method in which a mixture of a dispersion of a polymer electrolyte and a metal salt for a catalyst is coated on a polymer electrolyte membrane, an electrode material is hot-pressed on the coating layer, and then the metal salt is reduced; or a method in which, after reducing the metal salt in the mixture including the polymer electrolyte, the mixture is coated on the polymer electrolyte membrane, and the electrode material is hot-pressed on the coating.
Japanese Examined Patent Publication No. Hei 2-48632 proposes a method in which, after molding a porous electrode, a solution of an ion exchange resin is sprayed on the electrode, and then the electrode and ion exchange film are hot-pressed. Japanese Laid-Open Patent Publication No. Hei 3-184266 proposes a method in which a powder prepared by coating the surface of a polymer resin with a polymer electrolyte is mixed into the electrode, while Japanese Laid-Open Patent Publication No. Hei 3-295172 proposes a method in which a polymer electrolyte powder is mixed into the electrode. Japanese Laid-Open Patent Publication No. Hei 5-36418 proposes a method in which an electrode is produced by mixing a polymer electrolyte, a catalyst, a carbon powder and a fluorocarbon resin together and making the mixture to form a film.
In addition, the specification of U.S. Pat. No. 5,211,984 reports a method which comprises preparing an ink-like dispersion composed of a polymer electrolyte, a catalyst and a carbon powder using glycerin or tetrabutyl ammonium salt as a solvent, applying the dispersion on a film of polytetrafluoroethylene (hereinafter referred to as “PTFE”) and then transferring it onto the surface of a polymer electrolyte membrane; or a method which comprises converting a proton exchange group of a polymer electrolyte membrane to a Na type, applying the ink-like dispersion on the surface of the membrane, and heating and drying the coat at a temperature not lower than 125° C. to convert the exchange group converted to the Na type again to the H type.
Moreover, in order to realize a high output current that is a characteristic feature of the polymer electrolyte fuel cell, it is important to increase the gas permeability/diffusing performance by forming reaction gas supply channels (gas channels) in the electrode catalyst layer. Therefore, attempts have been made to form the gas channels by adding a water repellent agent such as a fluorocarbon resin to the electrode catalyst layer. For instance, in Japanese Laid-Open Patent Publication No. Hei 5-36418, a catalyst layer is formed by dispersing and kneading a carbon powder carrying a catalyst and a PTFE powder into a dispersion of a polymer electrolyte. In Japanese Laid-Open Patent Publication No. Hei 4-264367, an electrode is formed using a mixed liquid of a carbon powder carrying a catalyst and a PTFE colloid. Further, according to J. Electroanal. Chem. No. 197 (1986), p. 195, a gas diffusion electrode for an acidic electrolyte is formed by mixing a carbon powder subjected to water repellent treatment using PTFE with a carbon powder carrying a catalyst. On the other hand, according to the specification of U.S. Pat. No. 5,211,984, the catalyst layer of the electrode is fabricated using only a polymer electrolyte, a catalyst and a carbon powder without using a water repellent agent as mentioned above.
With the above-mentioned techniques, however, since a carbon powder carrying a catalyst and a water repellent agent such as a fluorocarbon resin, or a carbon powder subjected to water repellent treatment are added simultaneously to a polymer electrolyte solution, a large amount of the polymer electrolyte adheres to the water repellent agent or the carbon powder subjected to the water repellent treatment, and accordingly the degree of contact between the polymer electrolyte and the catalyst becomes uneven, resulting in a drawback that a sufficient reaction area is not secured in the interface between the electrode and the polymer electrolyte membrane. Besides, when the electrode is fabricated using only a carbon powder carrying a catalyst and a polymer electrolyte, there is a drawback that the cell voltage at a high current density is unstable due to flooding caused by generated water.
As means for solving these drawbacks, Japanese Laid-Open Patent Publication No. Hei 8-264190 discloses a method in which a colloid of a polymer electrolyte is produced and adsorbed to a catalyst powder. However, this method has a problem that, when a conventionally used perfluorocarbon sulfonic acid ionomer having a polymerization degree of about 1000 is used as the polymer electrolyte, it is impossible to cause a noble metal catalyst present in pores smaller than colloid particles to function effectively.
Other examples of electrodes improved by focusing on the pores of the catalyst layer like the above, Japanese Laid-Open Patent Publication Nos. Hei 8-88007, Hei 9-92293, and Hei 11-329452 specify 40 to 1000 nm and 30 to 1000 nm as the optimum values of the pores of the catalyst layer. Further, as examples of electrodes improved by focusing on the pores in the primary particles of carbon particles, Japanese Laid-Open Patent Publication Nos. Hei 3-101057, Hei 9-167622, 2000-003712, and 2000-100448 specify that the threshold value for the pores in the carbon particles used for the catalyst layer is between 2.5 and 7.5 nm, not more than 8 nm, or not less than 6 nm.
Conventionally, one obtained by dispersing a generally used perfluorocarbon sulfonic acid ionomer in a solvent is generally called a polymer electrolyte solution. However, as disclosed in Macromolecules, 1989, No. 22, p.p. 3594–3599, for example, in a polymer electrolyte solution, the polymer electrolyte ionomer is just dispersed and is not dissolved in the solvent. Therefore, if the molecular weight of PTFE that is a main chain of the ionomer increases or if a polymerization degree of the ionomer increases, the particle size of the polymer electrolyte particles in the dispersion of polymer electrolyte increases.
On the other hand, when carbon particles gather, the gathered state forms an aggregate structure in which the primary particles bond together in a fused state, or an agglomerate structure in which the primary particles are simply intertwined physically and secondary. Carbon particles used generally in a fuel cell form a particulate structure called an agglomerate particle resulting from further gathering of the aggregate structures. At this time, if carbon particles with 10 to 50 nm primary particles and a large specific surface area of not less than 200 m2 are used, the pores in the agglomerate structure of the carbon particles become very small. Then, when a generally used perfluorocarbon sulfonic acid ionomer with a polymerization degree of about 1000 is used as a polymer electrolyte, the polymer electrolyte cannot enter the pores in the agglomerate structure and cannot come into contact with a catalyst metal in the pores, and therefore the catalyst cannot be used effectively.
In the structure of the catalyst layer of a conventional fuel cell, a perfluorocarbon sulfonic acid ionomer with a polymerization degree of about 1000 is used as the polymer electrolyte. Hence, optimization from the above-mentioned structural view point has not been made. In other words, in order to bring the catalyst in the pores and the polymer electrolyte into contact with each other for an increase of the reaction area, the state of the polymer electrolyte also needs to be optimized, and it is insufficient to just optimize the pores of the catalyst layer or the pores of carbon particles as in the conventional examples.